Gaseous ammonia has a Delta_f H degree = -45.9 kJ/mol and Delta_f G degree = -16

Question

Gaseous ammonia has a Delta_f H degree = -45.9 kJ/mol and Delta_f G degree = -16.5 kJ/mol. (Data at 298K) Is the gas phase synthesis of ammonia ( N_2 (g) + 3 H_2(g) rightarrow 2 NH_3 (g) ) spontaneous at 298K? Under which conditions would you expect to maximize production of NH_3: high T, high P high T, low P low T, high P low T, low P Justify your answer. At temperatures less than 500K the yield of gaseous ammonia is vanishingly small. Industrial production is usually carried out at temperatures around 700K. Justify this in terms of something we have studied within the past month. Would it help to find a catalyst? Why or why not?

Explanation / Answer

a)

the reaction is

N2 + 3H2 ----> 2NH3

we know that

dGo = dGfo products - dGfo reactants

so

dGo = 2 x dGfo NH3 - dGfo N2 - 3 x dGfo H2

dGo = ( 2 x -16.5) - 0 - 0

dGo = -33 kJ/mol

we know that

for a reaction to be spontaneous

dG < 0

So

the reaction is spontaneous at 298 K

b)

now

dH = - ve

is is exothermic reaction

so

the reaction can be written as

N2 + 3H2 ---> 2 NH3 + heat

now

according to Le Chatlier principle

the equilibrium will shift in direction to counter the change

so

now at low temperature

it means heat is reduced

so

to counter the change the reaction will produce more heat

so

the reaction will move towards the products side

so

more NH3 will be produced at low temperatures

now

N2 + 3H2 --> 2 NH3

at equilibrium

there is more pressure in reactants than in products

so

according to Le Chatlier principle

if the pressure in high , the equilirbium will shift to the right side to decrease the pressure

so

more NH3 will be produced at high pressure

so

the answer is

iii) low T , high P

c)

because of low temperatures

the reactant molecules with very little kinetic energy

so

collisions between the hydrogen and nitrogen will be very feeble and reaction will not occur, thus

resulting in no ammonia formation

d)

So a catalyst should be selected which will have high activity at 700 K

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